1. Field of the Invention
The present invention relates to a water base ink for ink-jet recording which makes it possible to reduce the blurring on the paper surface, which is excellent in drying performance, and in which a pigment is stably dispersed.
2. Description of the Related Art
In the ink-jet recording method, ink droplets are formed by the ink discharge method including, for example, the electrostatic attraction method, the method in which mechanical vibration or displacement is applied to the ink by using a piezoelectric element, and the method in which bubbles are generated by heating the ink to utilize the pressure generated thereby. All or a part of the ink droplets are adhered to a recording medium such as paper to perform the recording.
The ink-jet recording method remarkably comes into widespread use in recent years, because the method involves no process of, for example, development and fixation, and it is easy to realize the color printing. Recently, the advance is rapidly made for the realization of the high speed and the highly fine definition in the printing performed by the ink-jet printer. Further, for example, the color printer is dominantly used, which makes it possible to perform the printing on the regular paper.
In such circumstances, for example, the following high performance is required for the water base ink for ink-jet recording to be used for the ink-jet recording method. That is, the jetting operation can be stably performed without causing any clog-up in the ink flow passage and at the tip of the head of the ink-jet printer. The image and the coloration are vivid. Neither fading nor discoloration occurs after the formation of the image. In particular, it is more demanded in recent years to perform the recording on the regular paper rather than on the exclusive ink-jet paper in consideration of the running cost and the environment. It is required that a vivid image without any blurring is obtained even when the recording is performed on the regular paper.
The water base ink for ink-jet recording is roughly classified into the dye ink and the pigment ink. The dye ink is an ink which has been used from the beginning of the development of the ink-jet recording method. The dye ink is excellent in variety of numbers of colors and in vividness of hue or tone. However, the dye ink has such a drawback that the dye ink is inferior in water resistance and weather resistance, because the dye itself exhibits the water solubility. On the other hand, the pigment ink is satisfactory in water resistance and weather resistance. However, the pigment ink has, for example, the following drawbacks. That is, the production steps are complicated, because it is necessary to finely disperse the pigment in water by using a dispersing machine. The pigment, which has been dispersed in water, is aggregated and precipitated in a time-dependent manner. Further, the pigment ink is inferior in vividness of hue or tone.
Researches have been made in order to improve the drawbacks of the dye ink and the pigment ink. In recent years, the pigment ink has been especially improved remarkably. Pigment inks, which are excellent in dispersion stability and which hardly cause the aggregation and the precipitation, have been developed. Such pigment inks include, for example, those in which the dispersing agent is improved and those in which the surface of the pigment is treated so that the electric or physical repulsion is generated without using any dispersing agent. The improved dispersing agent, which is used in the former pigment ink, is often a high molecular weight compound. In this case, the high molecular weight compound is adsorbed to the surface of the pigment, and the pigment is prevented from the aggregation by the steric hindrance action. On the other hand, the pigment, which is used in the latter pigment ink, is called “self-dispersing type pigment”. In this case, for example, a group, which has the electric charge, is added to the surface of the pigment. The pigment is prevented from the aggregation by, for example, the electric repulsion.
When the recording is performed on the regular paper by using the water base ink for ink-jet recording as described above, for example, the following problems arise. That is, the feathering, which mainly causes the blurring of the recorded image, appears in some cases, and the ink is adhered to the hand in other cases when the paper is touched after the recording, because of the unsatisfactory drying performance on the paper surface. The feathering is such a phenomenon that the ink is spread nonuniformly along the surface fibers of the paper and the edge is notched. In particular, when the recording is performed on the recycled paper composed of various paper components, the problems concerning the feathering and the drying performance on the paper surface appear more conspicuously because of different velocities of permeation of the ink into the fibers.
In the case of the general water base ink for ink-jet recording, a water-soluble organic solvent such as glycol ethers, which is referred to as the moistening agent or the permeating agent, is added in order that the permeability into the paper is controlled to solve the problems concerning the feathering and the drying performance on the paper surface. However, it is difficult to realize both of the reduction of the blurring on the paper surface and the drying performance. Further, the pigment ink, which is based on the use of the dispersing agent, has involved such a problem that the dispersion stability of the pigment is destroyed, resulting in the aggregation or the increase in viscosity.